Understanding dFIRMs necessary for intelligently mapping hazards

Editor's note: Flooding is not only the most pervasive natural disaster in the United States, but it also causes the greatest monetary losses. According to the National Oceanic and Atmospheric Administration, during the 20-year period between 1983 and 2003, flooding caused an average of $4.6 billion in damage, more than any other severe weather condition. Data locating and characterizing floodplains accurately are vitally important to emergency managers, planners, building inspectors, insurance agencies, and others. Part of the Federal Emergency Management Agency (FEMA) Map Modernization program is making digital floodplain data available in GIS formats.

Detail from the same area shown on a q3, dFIRM, and paper FIRM for the same area.

Digital Flood Insurance Rate Maps (dFIRMs) have improved the quality of floodplain maps and put information in the hands of people who need it. FEMA's Map Modernization program has brought floodplain management into a new age. Data from this new source has been incorporated into ArcIMS sites across the nation and helps people better determine flooding risks.

Over the next 40 years, it has been predicted that the Map Modernization program will save $160 billion. Congress has created a program that directly benefits citizens while mitigating the potential loss of lives and property due to flooding and has also given GIS users in many sectors and industries better quality data that is easier to use. The accompanying article, "Floodplain Mapping UpdatesDigital format now firmly established as data source for GIS," gives background on the development of dFIRMs.

Use the Symbology tab of the Layer Properties dialog box in ArcMap to symbolize flood zones.

DFIRM datasets can contain as many as 30 tables. However, all dFIRMs have a polygon layer, called S_FLD_HAZ_AR, that delineates flood zones. To identify all the flood zones in a specific dFIRM, add S_FLD_HAZ_AR to a map. Symbolize this layer based on the Value Fields FLD_ZONE and FLOODWAY by using Categories > Unique Values, Many Fields and pressing Add All Values. The zone descriptions can be lengthy, so edit the label to something shorter but descriptive.

These lengthy descriptions are for insurance purposes. All A and V Zone areas have a 1 percent annual chance of flooding, but areas designated AE or VE zones also contain detailed hydraulic analyses. Recent topographic, hydrologic, and hydraulic information was used to calculate the 100-year base flood elevation so the floodplain boundary is more accurately defined. Owners of buildings located within an A or V Zone are required to carry flood insurance. Insurance agents calculate the appropriate flood insurance premium based on the type of A or V Zone, date of construction, and elevation of the building's lowest floor.

Businesses can use dFIRM data to determine if facilities are located in flood zones. Base flood elevations can be extrapolated using the S_BFE layer. Avoiding a chemical release in a flood may be as simple as moving dangerous substances to a higher storage location. This map shows facilities associated with chemical industries in Washougal, Washington.

All A zones can be set to the 100 Year Flood Overlay fill symbol found in the default Esri symbology set. However, to make the symbology match FEMA's official specification, use a fill pattern of cyan dots with a dot diameter of 0.020 inch and a spacing of 0.030 inch offset between rows 0.025 inch for all A, AE, AO, AH, AR, A99, V, and VE zones. These standards are published in Appendix K of the Guidelines and Specifications for Flood Hazard Mapping Partners (April 2003), which can be downloaded in PDF format from the FEMA Web site.

Understanding the tables and layers in dFIRM datasets is the key to successfully mapping floodplains. A detailed listing of the contents of a dFIRM dataset can be found in Appendix L of the Guidelines and Specifications for Flood Hazard Mapping Partners. Some of the most useful and common layers are briefly described here.

Parcel maps maintained for tax purposes by the assessor's office can help determine if lots are in or near a flood zone.

Flood zones can be displayed as areas using the polygons in the S_FLD_HAZ_AR layer or as boundary lines using the S_FLD_HAZ_LN layer. River depths, where sampled, are available from cross sections in the layer S_XS. Base flood elevation lines can be found in the S_BFE tables. If a National Flood Insurance Program (NFIP) member community contains specially protected coastal barrier resource systems, this information will be contained in a layer called S_CBRS. There are also lookup tables and tables of useful information, such as Study_Info and L_Comm_Info. The Study_Info file supplies datum and projection information for a dFIRM. The L_Comm_Info file has map repository and history information. Table 1 lists some common dFIRM layers and explains how these layers are used.

DFIRMs provide more data than the q3 data and supply all the data that was previously available only on paper maps. Instead of incorporating all data in a single layer, dFIRMs feature multiple layers and coverages that can be more easily combined with local data on features, such as vacant parcels, critical infrastructure, demography, and hazardous materials sites, and used for analysis. This new format gives GIS users flexibility in creating custom floodplain maps that meet the needs and specifications of various organizations. Emergency managers, city planners, communities, and businesses can create disaster plans that help mitigate the effects of floods and maintain continuity of operations in times of emergency based on the integration of dFIRMs and local GIS data. Look at some of the possibilities in the series of maps that follow.

What's a 100-Year Flood?
A 1 percent annual chance of flooding is sometimes referred to as the 100-year flood or base flood. This means that a given area has a 1 percent chance of experiencing a flood equal to or exceeding the boundary of the base flood elevation line. This doesn't mean that this area can't flood more oftenjust that the probability of that happening is not as high. Statistically, a house has a 26 percent chance of being damaged by a flood during the life of a 30-year mortgage as compared to a 9 percent chance of being damaged by a fire. Flooding causes more damage than any other natural disaster in the United States.

Photograph by Marvin Nauman/FEMA
When the Snohomish River in Washington breached its levee in November 2006, surrounding towns were flooded.

NFIP member communities and nonfederal sources, such as state or regional agencies, are the first choice for basemap data for dFIRM production if they meet FEMA criteria. Digital orthophoto quadrangles produced by the U.S. Geological Survey are the second choice for base data and the default dFIRM basemap data if suitable community data is not available. FEMA mapping partners use software such as Esri Production Line Tool Set (PLTS) to create consistent and reliable maps. Symbology, annotation, quality control, and even the north arrow standards are enforced for every map using PLTS. The Job Tracking for ArcGIS (JTX) component allocates resources and tracks the status and completion of jobs. Project managers can check the status of any given map in production. Completed work is sent to FEMA's Map Service Center at msc.fema.gov where it is immediately available to the
public.

Making the data available is only part of FEMA's mission. GIS technicians now have new data that can be used for analysis. At this time, the program has modernized more than 50 percent of the maps in the United States with more scheduled for the coming years with areas that have large populations and are at a greater risk for flood hazard given the highest priority. FEMA has already anticipated the need for updating and maintaining this digital information. For more information on NFIP and online tutorials on using dFIRM maps, visit www.fema.gov/plan/prevent/fhm/mm_main.shtm.

About the Author

Nick Delmedico

Nick Delmedico has worked for FEMA since 1995. As a GIS technician, he supported disaster operations across the nation, including the 9-11 attacks. He holds a master's degree in business administration and has been a Microsoft Certified Solution Developer. He developed and deployed ArcPad applications on mobile devices and helped set up FEMA's first fielded Enterprise GIS Server in Orlando, Florida, following Hurricane Charley in 2004. He currently works on the Map Modernization program at FEMA Region X in Bothell, Washington. Contact him at Nicholas.Delmedico@dhs.gov for more information on the Map Modernization program and the National Flood Insurance Program. Request a free digital copy of his publication dFIRM Quick Reference Guide, a useful reference for anyone using ArcGIS and dFIRM data.

Layer

Use

S_FIRM_PAN

This layer contains data and spatial information about the FIRM panel area.

S_GEN_STRUCT

This required layer for floodplain maps shows hydraulic structures such as levees, channels, and sea walls.

S_Label_Pt

This point data references the best location for a given feature label. The attributes contain not only the label itself but also a label type field that helps set the correct annotation and a field with the correct angle of rotation. If the label contains a leader line, it will be in the S_Label_Ld layer.

S_PLSS_Ar

For areas where U.S. Public Land Survey System (PLSS) boundaries were shown on FIRMs, this layer helps line up maps with other layers. It is also available as line data in S_PLSS_Ln.

S_Pol_Ar

This layer contains spatial and attribute data about political areas within the study area, such as parks, forests, and military and tribal lands, and is also available as lines in the S_Pol_Ln coverage.

S_Transport_Ln

Linear basemap transportation features, such as roads, railroads, and airports, on this layer will line up other layers in the dFIRM.

S_Wtr_Ar and S_Wtr_Ln

River systems change over time. These polygon and line layers help water coverages line up properly. These layers are excellent for displaying vector data where digital orthophotos are not available.